Hello, and welcome!
The answer to this can depend on the situation. Is it business, personal, how many people are present, does the person entering the room know anyone in there?
Typically, the person entering the room, would introduce themself when they walk in, if s/he is not known to those in the room. However, someone who may know the new person, could greet them first, and then introduce them to the rest of the people.
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